Time of day information is conventionally sent to specialized receivers designed to receive synchronized RF time signals, such as Global Positioning System (GPS), WWVB, or Loran C signals. Such receivers can be situated at different locations within a large geographic area, and may have a real-time clock which is updated to an accurate time of day responsive to received time signals. A real-time clock is defined as an electronic clock, such as timing circuits similar to those found in digital watches, which maintains the time of day in hours, minutes, and seconds, and in some cases in fractions of a second. The clock provides an output containing time of day information. These receivers are relatively large, costly to manufacture, and may have bulky external antennas. Further, they cannot reliably receive RF time signals when located within buildings absent an extension of their external antennas. Therefore, it is desireable to provide a system for providing remote receivers with time of day information in which the receivers are relatively small, less costly to manufacture, and have real-time clocks that are maintained accurate based on time of day information sent via signals which may be received both inside and outside of buildings. Such systems have been proposed utilizing a paging receiver (also called a pager) as discussed below, but either fail to provide clocks with accurate time of day information or are based on a non-conventional design which increases manufacturing costs, or both.
In the paging industry, page messages are encoded into signals transmitted over a paging channel by a paging provider system. A paging channel is defined as a dedicated frequency for transmission of RF paging signals. Encoding is based on a paging protocol, such as POCSAG or GOLAY. These protocols are not designed to provide synchronized time signals, such as defined above, because they are limited to encoding alpha-numeric or numeric page message data. For example, POCSAG encodes page message data into eight frames of a data patch which is periodically transmitted in signals via a paging channel to pagers. Pagers are designed to decode received signals from the paging channel to obtain the encoded page message data therein. Within the encoded data, each pager is uniquely identified by a pager address, called herein a capcode. Only one of the frames of the data patch may contain encoded data with a pager's capcode. Consequently, only decoded page messages having a pager's specific capcode will be displayed to the pager's user.
These pagers often have a real-time clock for providing time of day information for time stamping received page messages. Generally, a time stamp is stored in the pager with each of received page message. Such real-time clocks may also be used to control turning on the pager's receiver responsive to preset periods during the day when signals may be sent over the paging channel to the pager. By limiting the time when the pager's receiver is active, pager battery power may be conserved. Thus, maintaining the accuracy of real-time clocks is an important consideration for pagers.
Several approaches have been proposed to maintain the accuracy of the real-time clock of a pager. In a first approach a page message with data defining the time of day is encoded into signals and transmitted via a paging channel to a pager. The pager is programmed to detect the page message as containing the correct time of day, and then updates its clock accordingly. One problem with this approach is that the received time of day is inaccurate and thereby cannot provide the correct time of day. This is due to the delays incurred before the page message is actually sent over the paging channel to the pager. This delay is based on the time the page message waits in a queue (e.g., memory buffer) with other page messages prior to being encoded, and the time it takes for encoding the page message. This delay is variable and can range for example from several seconds to minutes. Further, additional delays are incurred after the page message is sent over the paging channel due to the time lag based on the distance the signal must travel (e.g., 5 .mu.seconds per mile) in the paging channel to the pager, and the time for decoding the page message at the pager. Another problem with this first approach is that it is very time consuming because a separate page message with the time of day must be sent to each pager. This is because each pager has a separate identifying capcode, as described above, and conventional protocols require that each page message must specify a specific capcode to uniquely identify the receiving pager. Accordingly, this first approach is unacceptable.
A second approach to sending time of day information to a pager involves sending control information to a pager having the time of day. For example, U.S. Pat. No. 4,713,808 describes a wrist watch pager which receive control packets with time of day information for updating the watch time. These control packets are separate from encoded page messages and would avoid the variable delay described above. The problem with this approach is that the control packets utilize a non-conventional paging protocol format. This increase the cost of implementing control packets in a typical pager design. Thus, this second approach is also unacceptable.
A third approach is to send time synchronization signals or pulses without data defining the time of day. Time synchronization signals are sent to a pager at predefined times of the day called on-time points, such as at midnight, synchronous with a UTC. After the pager receives a time synchronization signal or pulse, it updates its clock to the predefined time of day. One problem with this approach is that the on-time points are limited to those pre-defined with the UTC in the pager. Further, the updated clock will be inaccurate because time delays due to the distance the time synchronization signals must travel to the pager and processing time at the pager to decode the signals are not accounted for. These time delays may for example be up to several seconds. Consequently, this third approach is unacceptable where fine time resolution (e.g., resolution under 1 minute) is needed.
A fourth approach for providing time of day information to a pager is described in U.S. Pat. No. 4,845,491, issued Jul. 4, 1989, which discloses a real-time clock in a pager for time stamping received messages. The pager clock is updated based on two received time messages. Each time message has data defining the time of day (day, hour and minute) without any specified on-time point in the message. The second of these time messages also has data defining a time correction, i.e., the difference between the time encoded in the first time message and the actual time of day when first time message was broadcasted. The time messages are each encoded into signals sent via a paging channel to a pager. The clock of the pager receives both time messages and is then updated to a corrected time of day by adding the time of day from the first time message with the time correction of the second time message, and the difference between the time of day of the pager clock when the first and second time messages were received by the pager as determined by its clock. A complex multi-recipient grouping scheme based on a non-conventional encoding protocol is also described by U.S. Pat. No. 4,845,491 to permit more than one pager to receive page messages.
There are several problems with this fourth approach. One problem is that the multi-recipient grouping scheme although allowing more than one pager to receive the time messages is complex and costly to implement because it depends on a non-conventional paging protocol. Another problem is that the updated clock time at the pager will be inaccurate for time resolutions under a minute because it fails to account for the time lag based on the distance a signal must travel in the paging channel to the pager, and the time for decoding the first and second time messages at the pager to detect them in signals in the paging channel. The time messages must be detected prior to determining their time of receipt by the pager clock. Further, the pager clock cannot be accurate to fine time resolutions since there is no specified on-time point or mark defining the exact point where the time of day in the time message is in reference to.
In light of the above four approaches, it is therefore desireable to provide a system for providing multiple paging receivers with accurate time of day information in which each of the receivers has a real-time clock that is accurately updated to a time of day with fine time resolution by accounting for all the above defined time delays. Moreover, it is desireable that the clock at each receiver is updated responsive to page messages received, in signals via a paging channel, having data encoded using conventional protocols and defining therein an on-time point.